Highly pulverulent silica is one the main components of Vacuum Insulation Panels (VIPs) exhibiting high performance thermal insulation. When they are perfectly dry, pyrogenic silica powders are one of best materials for thermal performance for this application. However, owing to their high affinity for water, vapor adsorption capacity and ageing under various conditions must be considered. The main objective of the present study is to test the behaviour of HDK T30 (Wacker) silica powder at different temperatures and in the presence of a controlled humid atmosphere. The stability of the microstructure has also been considered under the same ageing conditions. Three ageing conditions of temperature and relative humidity (RH) have been considered in this work: 20 degrees C-44%RH. 35 degrees C-80%RH and 60 degrees C-80%RH. Silica powder samples have been compressed under pressure as pellets of 0.2 g cm(-3) density. With the exception of an increase in the rigidity of samples, no modifications of the silica properties were observed after storage at 20 degrees C-44%RH. However. at 35 degrees C-80%RH and 60 degrees C-80%RH a hydroxylation of the surface occurred leading to an increase in the water adsorption capacity of the material. Furthermore, an increase in the rigidity modulus greater than that recorded for the 20 degrees C-44%RH condition and a 20% decrease in the specific surface area have been simultaneously observed. This result is interpreted in terms of microstructural modifications occurring under highly humid atmospheres. (C) 2008 Elsevier B.V. All rights reserved.